Windows 98 contains support for dual-modems,
assuming that your ISP also supports it.
You can also put one LAN card into the PC with the two modems,
and one LAN card into each of the other PCs,
and then you can "network" among the PCs,
and with software like WinProxy (http://WWW.WinProxy.COM)
to provide a "gateway" between your "local" network and your ISP.